The contracile proteins are now thought to be a universal component of all cells. We still know very little about the mechanisms involved in switching on and off contractile proteins in smooth muscle and non-muscle cells. We are studying the myoepithelial cells of the mammary gland. The reason that the myoepithelial cells serve as a unique system to study excitation contraction coupling is that they are non-innervated cells which respond to only oxytocin and oxytocin-like analogues. Since they are not stimulated by a wide range of compounds we feel it would be easier to determine the sequence of events between excitation of the cells and contraction. We are assaying the myoepithelial cells for myosin light chain kinase and phosphatase activity. The mammary glands of both lactating animals and animals whose mammary glands have regressed are being tested. The roles of oxytocin, calcium and magnesium are being examined. The phosphorylation of the myosin light chain is being correlated with the tension development of mammary tissue strips.